IPv6 is the latest version of the Internet Protocol (IP) that is the basis of how the internet is built and how it runs.
The internet was originally designed using IPv4, but after several years of enormous growth, it started to look like the world would run out of IP addresses. In 1998, IP version 6 (IPv6) was standardized. Simply put, version 6 gives the internet more IP addresses and additional features.
When you lease a static IPv4 address, it will typically have a "derived IPv6 address" associated with it, so there is no need to order anything additional. If your modem is compatible, then you just have to enable IPv6 on your modem. All of the newer modems from CenturyLink are IPv6-compatible.